US 3246450 A
Description (OCR text may contain errors)
April 1966 s. A. STERN ETAL 3,246,450
RECOVERY OF HYDROGEN Filed June 9, 1959 n INVENTORS M2 $.ALEXANOER STERN PAUL H. MOHR PAUL J. GARE/S ATTORNE United States Patent O York Filed June 9, 1959, Ser. No. 819,077 9 Claims. (CI. 5516) This invention relates to the recovery of hydrogen gas from admixture with other gases. More specifically, it relates to a process of and apparatus for recovery of hydrogen from a gas mixture containing at least one of the following gases: carbon monoxide, argon and gaseous hydrocarbons.
Hydrogen has found important application in modern technology, and for example is being used extensively by the chemical and petroleum industries. Even larger quantities may be required in the future for the propulsion of missiles and by the steel industry for the direct reduction of iron ore,
The recovery of hydrogen from the type of gas mixtures in which it usually occurs is quite diflicult. Most often, it is desired to recover hydrogen from admixtures with low boiling gases such as carbon monoxide, argon and gaseous hydrocarbons and their separation by low temperature fractionation is difiicult and costly.
Separation by absorption in appropriate wash liquids is subject to similar disadvantages, especially if the operation is carried out at low temperatures.
Other known methods are even less feasible for commercial use, as for example mass or sweep diffusion. This method is based on the difference in the rates of diffusion of various gases in some inert, readily condensable medium, such as steam, and is characterized by very low separation factors.
It has been proposed to recover hydrogen by selective permeation through membranes, using thin, non-porous film forming materials such as polystyrene and ethyl cellulose. The chief advantage of such schemes is that the selectivity of these materials for hydrogen is so low that an extremely large number of permeation stages are required to achieve the desired degree of separation. This not only entails a prohibitively high investment cost for the necessary equipment, but also involves extremely high power consumptions to operate the system.
Finally, it has been proposed to recover hydrogen by difiusion through palladium. Although the selectivity of this metal for hydrogen is very high, it is susceptible to poisoning by components of many hydrogen-containing industrial gas mixtures. In addition, palladium is very expensive.
A principal object of the present invention is to provide-a new and novel system for separating hydrogen from gas mixtures which is more efficient and less costly than heretofore proposed systems;
Another object is to provide a system for recovery of hydrogen from low-boiling gas mixtures which operates on the principle of selective permeation, and which requires a relatively few number of stages to attain a desired degree of separation.
A further object of this invention is to provide a selective permeation system for recovery of hydrogen from low-boiling gas mixtures, in which the permeable membranes afford a relatively high separation factor combined with a relatively high permeability.
These and other objects and advantages of this invention will be apparent from the following description and accompanying drawings in which:
FIG. 1 is a view in vertical cross section of an exemplary permeation cell construction employing the principles of the invention; and
3,246,450 Patented Apr. 19, 1966 FIG. 2 is a schematic flow diagram of several permeation cells arranged in a series relationship for practicing one embodiment of the invention.
The present invention is predicated on the discovery that the rate of permeation of hydrogen through thin, non-porous, iluorinated plastic material is much faster than that of the other low-boiling gases which comprise the mixtures normally containing hydrogen. The term fluorinated material as used herein is to be understood as including both partially and totally fluorinated materials. Among the membrane materials which have been found particularly suitable for practicing this invention are polytetrafluoroethylene, polytrifiuorochloroethylene and a copolymer of vinylidene fluoride and hexafluoropropylene known commercially as Viton A.
The selectivity of membranes for films for a given gaseous component in a binary mixture can be expressed in terms of a separation factor, which is defined as the ratio of the penneability constants of the pure components of the mixture. Table I lists the separation factors attainable for certain binary mixtures using membranes formed from previously proposed materials as well as the materials of the present invention.
TABLE I.-SEPARATION FACTORS ATTAINABLE BY VARIOUS MEMBRANES AT 30 C.
It is apparent from this table that fluorinated plastic membranes afford remarkably high separation factors for hydrogen, as compared with heretofore used organic membrane materials. For example, the separation factor for the reported fiuorinated membrane compounds is 1.5 to 3.4 times that of the prior art materials for a hydrogen-methane system and 1.2 to 2.9 times for a hydrogen-carbon monoxide system.
The suitability of fluorinated material membranes for use in the system of the present invention is further enhanced by their high absolute permeability, as well as a high selectivity for hydrogen. A high absolute permeability is of importance since it affords a measure of the total amount of gas that can be processed in a given time through a given film area. Although a membrane might be highly selective, if it has a low absolute permeability, it may not be economically utilized for large scale separation because of the relatively large membrane area that would be required. The absolute permeabilities of the membranes of the present invention are reported as a function of temperature in Table II by means of appropriate mathematical expressions. The permeability constants are in terms of cubic centimeters (STP) of gas permeated per second through one square centimeter of membrane, one centimeter thick under a pressure drop of 1 cm. Hg. Also R, the universal gas constant, is given in calories/ K. mole, and T the absolute temperature is given in K.
TABLE II.ABSOLUTE PERMEABILITIES OF VAR- IOUS MEMBRANE MATERIALS cm. STP.cm.
sec. cm. cm.H g.
CH =(3.0 GXp. m
sec.01n .-em. Hg.
em 'STR-cm. sec.cm .cm. Hg.
PH,= 2.0 10 exp.
on =(1.1 10 exp. RT
0111. STP.cm. sec.cm. cm. Hg.
The high absolute permeability of fluorinated membranes is further enhanced by the fact that they are preparable in very low thicknesses such as 0.1 ml. and if properly supportedmay be installed and used under pressure without danger of fracture. In a system involving gas separation by per-meation through a non-porous membrane, it is desirable to use very thin membranes since the absolute rate of permeation varies inversely with the membrane thickness.
Another important consideration in providing a membrane material suitable for a gas recovery or separation system is chemical inertness to the gases which it will contact. The chemical inertness of fluorinated membranes is outstanding; these materials are known to be considerably less reactive than any of the known membrane materials such as ethyl cellulose and polystyrene. Furthermore, fluorinated membranes are not subject to poisoningby specific omponents of industrial gas streams, as for example are palladium membranes to sulfur-containing compounds.
A further important consideration is the physical stability and mechanical strength of membrane materials. The mechanical-properties of the fluorinated membranes of the present invention are excellent; these membranes have beenshown to retain their toughness and flexibility over wide ranges of temperature. By contrast, certain types ofpolystyrene are very brittle and cannot be used in a practical hydrogen-separation process. Fluorinated vmembranes are also characterized by their unusual thermal stability, being suitable for continuous service at 500 F.
Broadlythe process of the invention involves the steps of contacting-agas mixture containing hydrogen with one side of a thin, non-porous membrane comprised essentially of a plastic film-forming fluorinated material, and permeating a portion of the gas mixture through the membrane, this portion having a greater concentration of hydrogen than the gas mixture. The permeated gas is removed from the opposite side of the membrane.
It .isessential that the membranes of the invention be non-porous; that is, be free from pin-holes and other defects destroying their continuity. Discontinuity in the membrane large enough to allow gas .to leak through rather than permeate through the body of the membrane almost completely destroys the selectivity of the membrane for hydrogen. In this connection, it is emphasized that the gas recovery system of the present invention does not operate on the same principle as separation of gases by diffusion through porous media wherein advantage is taken of the difference in the rates of diffusion of the component gases through the pore structure. The present invention involves a permeation of the gas mixture through the body of the membrane rather than through pores present therein, and depends upon the fact thathydrogen permeates-through the membrane in this mannerat a considerably faster rate than many other gases. The presence of pores in the membranes virtually destroys its selectivity by permitting large quantities of relatively unseparated gas to leak through.
As previously discussed, the selectivity of fluorinated membranes for hydrogen in mixtures with low-boiling gases such as carbon monoxide and gaseous hydrocarbons is remarkably and unexpectedly high. The reasons for this characteristic are not fully understood, but the permeation of gases through membranes is known to be of a physicochemical nature and involves a solution .of the gases at one interface of the membrane, the diffusion of the gases through and across'the membrane, and the evaporation-of the gas at the opposite interface of the membrane. The differences in the rates of permeation of certain gases are believed to be causedmainly by diflferences in the solubility and rate of diffusion of these gases in a given membrane.
The rates of permeation of diiferent gases through a membrane are a measure of the absolute permeability of the membrane to these gases, and can beexpressed by the following simple form of Ficks diffusion law where q is the rate of permeation, P is the permeability constant, A isthe area of the membrane, Ap is the partial pressure gradient across the membrane, and t is the thickness of the membrane. Practical considerations require that the area of the membrane be kept at a minimum Consequently, a high rate of permeationcan be achieved if the permeability constant and the partial pressure gradient are large and the membrane is made as thin as possible.
The magnitude of the pressure gradient which can be applied across the membrane depends on such factors as the pressure of the feed gas mixture, the number of recovery or separation stages, the cost of compression, and the rupture limit of the membrane. The membrane itself does not have to sustain the entire pressure gradient by itself, but may be supported for example by porous metals or ceramic materials or by a metallic mesh or screen. The membranes of the present invention should be of a thickness less than about 5 mils, and preferably less than about 1 mil.
It has also been unexpectedly discovered that temperature has an important effect of the selectivity of fluorinated membranes for hydrogen, as illustrated in the following Tables III through V.
TABLE IIL-SEPARATION FACTORS FOR POLY- TETRAFLUOROETHYLENE AT VARIOUS TEM- PERATURES Temperature, Separation factor, 0. H2- H TABLE IV.SEPARATION FACTORS FOR PLAS- TICIZED POLYTRIFLUOROCHLOROETHYLENE AT VARIOUS TEMPERATURES Separation factor Temperature,
H2-CH4 1 2-00 TABLE V.SEPARATION FACTORS FOR VITON A AT VARIOUS TEMPERATURES Separation factor Temperature, C.
' HZ'CH; Hz-CO It is apparent from an examination of Tables III through V that the separation factors for fluorinated membranes markedly increases as the membrane temperature is decreased. However, it has also been discovered that the absolute permeability increases'with increasing temperature. Thus, for any given gas recovery system operating according to the present invention, the optimum permeation temperature will be determined by balancing the increased separation factors attainable at lower temperatures against the lower absolute permeability, as well as the investment and operating costs necessary to provide suitable equipment for the maintenance of the membranes and gas mixtures at the desired temperature level.
The advantages of the. present invention are clearly illustrated by the following examples:
Example I In this example, a synthetic gas feed mixture containing hydrogen and methane as a main impurity was supplied at a pressure of 120 p.s.i.g.;and contacted with a single .polytetrafiuoroethylene membrane having an area of 7.1 sq. in. and a thickness of lmil. The membrane was supported by porous paper mat on a metal screen. A portion amounting to about 7.1 of the 120 p.s.i.g. gas mixture permeated through the membrane at 30 C., and was removed from the opposite side thereof as product gas at 0 p.s.i.g. The non-permeating portion of the gas mixture was discarded, and the gases had the following molar compositions:
Component Feed Product the feed gas, decreasing the fraction of the feed gas mixture permeating through the membrane, or decreasing the permeation temperature.
Example II In this example, a synthetic gas feed mixture containing hydrogen and CO as a main impurity was provided at a pressure of p.s.i.g. and contacted with a single polytetrafluoroethylene membrane having an area of 7.1 sq. in. and a thickness of 1 mil. The membrane was supported by porous paper mat on a metal screen. A portion amounting to about 12.5% of the 120 p.s.i.g. synthetic gas mixture permeated through the membrane at 30 C. and was removed from the opposite low pressure side thereof at 0 p.s.i.g. The non-permeating portion of the synthetic gas mixture was discarded, and the permeated gases hadthe following molar compositions:
Component Feed Product H2 57. 4 s3. 0 CH 5. 8 2. 0 N2 10. 9 3. 4 co 25. 9 11. 6
Inspection of this data reveals that the hydrogen concentration in the product gas is about 1.4 times that of the feed gas.
Fluorinated membranes are commercially available both without and with a plasticizer, which is usually a low-molecular weight fraction of the membrane-forming polymer. The effect of plasticizer is, in general, to increase the absolute permeability of the membranes. The mechanism is not fully understood, although it is possible that the crystallinity of the membrane is affected by the plasticizer. That is,.the addition of a plasticizer to a fluorinated membrane may reduce its crystallinity and increase its permeability.
Referring now more specifically to the drawing, FIG. 1 is a view in vertical cross-section of an exemplary permeation cell construction employing the principles of the present invention, and includes gas-tight casing 10 having an inlet connection 11 for the feed gas mixture, an outlet 12 for the permeated gas product fraction, and an outlet 13 for the non-permeated waste gas fraction. A membrane support assembly 14 is provided, and may for example include wire screens fabricated in box-type assemblies 15 being supported at the upper end by a series of beams 16 extending longitudinally across the casing 10. Any desired number of beams 16 may be provided, and may for example be spaced at uniform intervals across the casing. The wire screen box assemblies 15 may be secured to. the longitudinal support beams by any suitable means, as forexample tack-welds or bolts. The number of wire box support assemblies 15 will be determined by the required membrane surface area, and, the wire boxes may for example be spaced at uniform intervals along the longitudinal length of the casing. If desired, the wire box support assemblies 15 may be internally reinforced to withstand the pressure differential across the membranes. Thin, non-porous membranes 17 of fluorinated plastic material are secured to the outer surfaces of the wire screen box support assemblies 15 by suitable means such as an adhesive compound or tacks, and for example may be wrapped around the wire screen boxes prior to insertion and securing in the casing 10.
p The hydrogen gas-containing feed mixture is introduced through inlet connection 11 prefreably at a positive pressure, and contacted with the high pressure or outer sides 18 of the fluorinated membranes 17. A portion of the feed gas permeates through the membranes 17 to the lower pressure side 19 thereof, and flows upwardly through the inside of the wire screen box assemblies 15 to the low pressure product gas manifold 20 of the permeation cell. This manifold may be gas-tightly separated from the high pressure side of the cell by sheets 21 extending transversely across the permeation cell between the ends of the wire screen box support assemblies 15. Similar transverse sheets 21 are provided to separate the product manifold 20 from the feed gas and waste gas sections of the cell. Transverse sheets 21 may for example be secured to longitudinal support members 16 by welded joints.
The low pressure permeated gas product fraction collected in manifold 20 is discharged from casing through outlet 12 for further use as desired, and the higher pressure non-permeated waste gas fraction is discharged through outlet 13.
Due to the high selectivity of the fluorinated membranes of this invention, substantial recoveries of hydrogen may be made in only one stage of permeation. In many cases, however, it is desired to obtain a higher concentration of the desired gas than is possible to obtain in one stage of permeation, as for example, where the desired gas is present in the feed gas mixture in a very small concentration. In such cases, a multi-s'tage system may be advantageously employed.
Referring now to FIG. 2, a system for recovery of hydrogen is illustrated which comprises five permeation stages, although any desired number of stages may be provided. Each stage has been designated by the letters A, B, C, D, and E, respectively, and in its simplest form comprises a casing 50 divided by a thin, non-porous membrane S1 composed of a fluorinated material, into a high pressure portion H and low pressure portion L. A perforated support 52, which may be a wire screen, is arranged on the low pressure side of the membrane to prevent its collapse when placed under pressure. A feed gas mixture which may contain a very low concentration of hydrogen is compressed by means of compressor 53 to any desired pressure, and contacted with the high pressure side of the membrane in the first permeation stage A. The opposite side of the membrane is maintained at some lower pressure such as atmospheric. A portion of the feed gas permeates throughthe membrane to the lower pressure side thereof, and has a higher concentation of hydrogen than the unpermeated portion of the feed gas.
The staging may be accomplished as follows: The unpermeated gas fraction is discharged from the high pressure portion H of first stage A through conduit 54 for further use or discharge to the atmosphere, as desired. The permeated portion of the original feed gas mixture is wtihdrawn from the low pressure side of .first stage A through conduit 55, recompressed by means of compressor 56, and delivered to the high pressure side of the second stage B by means of conduit 57. In the second stage B, the same process is repeated. That is, a portion of the entering gas stream permeates from the higher to the lower pressure side of the membrane, and the unpermeated portion is recycled from the high pressure side of second stage B through conduit 58. The gas permeating through the membrane of second stage B is withdrawn from the lower pressure side L through conduit 59, recompressed by compressor 60, and circulated to the high pressure side H of stage C by conduit 61. The same process is repeated in stages C, D and F, the unpermeated gas from the high pressure side of each stage being recycled to thepreceding stage, while the hydrogen-enriched gas from the low pressure side of each stage is circulated with recompression if desired to the high pressure side of the next succeeding stage for further enrichment.
As previously discussed, it has been discovered that the separation factor for hydrogen-containing gas mixtures obtained with fluorinated membranes increases with decreasing temperature, and also that the absolute permeability decreases with decreasing temperature. The present invention thus provides a method of and apparatus for advantageously using these characteristics in a staged permeation system, by maintaining succeeding stages at progressively decreasing temperatures. That is, the temperatures in succeeding stages with respect to hydrogen concentration, are maintained at progressively lower values. In this manner, the advantage aflorded by increased separation factor at decreasing temperatures is utilized, and the simultaneous decrease in absolute permeability is not limiting to the overall process since the mass of gas circulating in succeeding stages is progressively decreasing. This temperature adjustment may be ob- &
tained by any of the numerous methods known to the refrigeration art, and as illustrated in FIG. 2 is accomplished by means of refrigerating coils. Thus, coils 62 may be provided in the high pressure section H of stages B, C, D and E, and a refrigerating fluid such as ammonia, carbon dioxideiand the like may be circulated therethrough to cool the gas mixture flowing through the stages. The degree of cooling may, for example, be controlled by regulating the quantity of refrigerant circulating through the coils so that progressively more refrigerant would be circulated in succeeding stages. I
The same: objective may be accomplished by another embodiment of this invention without adjusting the temperatures of the various stages, by using fluorinated membranes of different compositions. Thus, the first or first few stages may utilize polytetrafluoroethylene membranes which exhibit a relatively high absolute permeability (see Table II) and reasonable separation factors (see Table I) for mixtures of hydrogen and the aforementioned lowboiling gases. Succeeding stages may, for example, utilize membranes of trifluorochloroethylene or a copolymer of vinylidene fluoride and hexafluoropropylene, which possess higher selectivity but lower permeability than polytetrafluoroethylene for hydrogen.
A hydrogen recovery system according to the present invention may be combined with known separation systerm. For example, the hydrogen content in the crude feed gas mixture could be raised to a desired level in one or several permeation stages, and the final separation of the hydrogen-containing product gas could be elfected by means of low-temperature distillation, solvent extraction, adsorption or other appropriate methods.
It is to be understood that the term non-porous membrane composed essentially of plastic, film-forming fluorinated material as used herein includes materials obtained by any method that will provide an effective amount of fluorine in the membrane. Such methods include deriving polymers from fluorinated monomers, and fluorinating a polymeric substance either before or after processing the substance into a membrane. Alternatively, a fluorinated plasticizing material may be incorporated in the membrane. Also, organic-inorganic membrane materials may be employed, as for example organosiloxanes, and these materials may be provided with the required amount of fluorine by the above-described methods. As a further variation, coatings of fluorinated materials may be applied on highly porous supports such as glass cloth and specially treated metals, as well as fluorinated films deposited in porous matrices.
Although the preferred embodiments have been described in detail, it is contemplated that modifications of the process and apparatus may be made and that some features may be employed without others, all within the spirit and scope of the invention as set forth herein.
What is claimed is:
1. A process for the recovery of hydrogen from a gas mixture comprising hydrogen and at least one other gas selected from the group consisting of carbon monoxide, argon, and gaseous hydrocarbons comprising the steps of contacting such gas mixture with one side of a thin nonporous membrane comprised essentially of a plastic, filmforming, fluorinated material selected from the group consisting of polytetrafluoroethylene, and a copolymer of vinylidene fluoride and hexafluoropropylene; permeating a portion of said gas mixture through the membrane, said portion having a greater concentration of hydrogen than the gas mixture; and removing the permeated gas from the opposite side of said membrane.
2. A process according to claim 1 for the recovery of hydrogen in which membrane has a thickness of less than about 1 mil.
3. A process for the recovery of hydrogen from a gas mixture comprising hydrogen and at least one other gas selected from the group consisting of carbon monoxide, argon, and gaseous hydrocarbons comprising the steps of flowing said gas mixture maintained under a predetermined pressure in contact with one side of a thin, nonporous membrane comprised essentially of a plastic, film- .forming, fiuorinated material selected from the group consisting of polytetrafluoroethylene, and a copolymer of vinylidene fluoride and hexafiuoropropylene; maintaining the opposite side of such membrane at a pressure lower than the pressure on the first-mentioned membrane side; permeating a portion of said gas mixture through the membrane from the higher to the lower pressure side thereof; said portion having a greater concentration of hydrogen than the gas mixture; and removing the permeated gas from the lower pressure side of said membrane.
4. A multi-stage process for the recovery of hydrogen from a feed gas mixture comprising hydrogen and at least one other gas selected from the group consisting of carbon monoxide, argon and gaseous hydrocarbons in which a plurality of permeation stages are arranged in series relationship, each stage comprising a high pressure side and a low pressure side being separated by a thin, non-porous membrane comprised essentially of a plastic, film-forming, fluorinated material selected from the group consisting of polytetrafluoroethylene, and a copolymer of vinylidene fluoride and hexafiuoropropylene, comprising the steps of: supplying the feed gas mixture under suit able pressure and contacting it with the high pressure side of a first permeation stage; permeating a first portion of said gas mixture through the first stage membrane from the high to the low pressure side thereof, said first permeated portion containing a greater concentration of hydrogen than the feed gas mixture; passing said first permeated portion of gas from the lower pressure side of said first permeation stage to the higher pressure side of a second permeation stage, and contacting such first permeated portion of gas with the second stage higher pressure side; permeating a portion of said first permeated portion of gas through the second stage membrane from the high to the low side thereof, said second permeated portion containing a greater concentration of hydrogen than said first permeated portion; and thereafter consecutively permeating portions of said second permeated portion of gas through succeeding permeation stages in an analogous manner until the desired concentration of hydrogen is obtained.
5. The process of claim 1 wherein the membrane material is a polymer of tetrafiuoroethylene.
6. The process of claim 1 wherein the other gas is methane.
7. The process of claim 4 wherein each succeeding permeation stage is maintained at a progressively lower temperature than the last preceding stage.
8. The process of claim 4 wherein the membrane of said first permeation stage has a higher absolute permeability and a lower separation factor than membranes of succeeding permeation stages.
9. The process of claim 4 wherein the membrane material is a polymer of tetrafluoroethylene.
References Cited by the Examiner UNITED STATES PATENTS 2,540,152 2/ 1951 Weller.
3,010,536 11/1961 Plurien et al.
3,019,853 2/1962 Kohman et a1.
3,172,741 3/1965 Jolley 55l6 OTHER REFERENCES Dow Bulletin: Sarah Resin F. 120, Coatings Technical Service, Dow Chemical C0., Plastics Dept, Midland, Mich. Nov. 1954, p. 5. Copy in Interference File 88833.
Separation of Gases by Plastic Membranes, by D. W. Brubaker and K. Kammermeyer, Industrial & Engineering Chemistry, Vol. 46, April 1954, pages 733-739.
Permeation of Gases through Solids, by F. J. Norton, Journal of Applied Physics, Vol. 28, Number 1, January 1957, pages 34 to 39.
REUBEN FRIEDMAN, Primary Examiner.
HERBERT L. MARTIN, GEORGE D. MITCHELL,
WALTER BERLOWITZ, WESLEY S. COLE,